Misalignment of the vertebrae in the lumbar region of the spine can result in chronic pain and other complications due to abnormal motion of the vertebrae, as well as nerve compression by the misaligned vertebrae. Misalignment can result from a number of different conditions, such as, e.g., osteoporosis, scoliosis, degeneration or herniation of a spinal disc, tumor, or vertebral fracture.
Misalignment of vertebrae can be corrected with a spinal fusion. In this procedure, two or more vertebrae are joined together, preventing any misalignment or abnormal motion. The vertebrae are fused by placing bone grafts between the vertebrae and allowing them to grow together. The fusion process may take 6-12 months. During this time, the vertebrae must be held motionless relative to one another or the fusion will be unsuccessful.
To hold the vertebrae motionless, they may be joined together by a metal rod that is attached to screws placed into each vertebra. The typical screw path is through a pedicle and into the vertebral body. The pedicle and outer portion of the vertebral body are composed of cortical bone, providing a solid support for the proximal two-thirds of the bone screw. The interior of the vertebral body, however, is composed of cancellous bone, which does not provide as strong of a support for the distal one-third of the bone screw.
Poor support in the cancellous portion of the vertebral body may contribute to toggling or migration of the bone screw. Toggling or migration may be caused by powerful in vivo forces applied to the metal rod joining the vertebrae. These biomechanical forces are the result of normal movement, but the rod must provide resistance against them for the fusion to be successful.
Additional factors, including patient age and osteoporosis, may negatively affect the interface between the screw and the bone. Expansion of the screw path through the pedicle, which may be caused by factors such as screw strippage or revision due to misalignment, may also weaken the bone-screw interface. A poor quality bone-screw interface may lead to pullout of the screw, as well as toggling or migration.
Accordingly, there is a need for bone screws that provide an enhanced bone-screw interface, especially in the cancellous region of the vertebral body. The enhanced interface reduces incidents of pullout and toggling.